Variable controlled oscillators (VCO) are categorized into two groups based on the type of waveform produced: harmonic oscillators and relaxation oscillators. Harmonic oscillators generate a sinusoidal waveform; whereas, relaxation oscillators generate a sawtooth or triangular waveform.
Harmonic oscillators usually consist of a resonator with an amplifier that replaces the resonator losses (to prevent the amplitude from decaying) and isolates the resonator from the output (so the load does not affect the resonator). Some examples of harmonic oscillators are LC-tank oscillators and crystal oscillators. Relaxation oscillators, on the other hand, are commonly used in monolithic integrated circuits (ICs) and can provide a wide range of operational frequencies with a minimal number of external components. Relaxation VCOs are tunable over a wider range of frequencies than harmonic oscillators.
A voltage-controlled capacitor is one method of making an LC oscillator vary its frequency in response to a control voltage. An LC circuit is known as a resonant circuit, tank circuit, or tuned circuit. These types of circuits comprise an inductor and a capacitor connected together so that they can act as an electrical resonator, which stores energy oscillating at the circuit's resonant frequency. LC circuits are typically used for generating signals at a particular frequency, or picking out a signal at a particular frequency from a more complex signal. LC circuits are an important component in many electronic devices, particularly cellular applications, used in circuits such as oscillators, filters, tuners and frequency mixers.
A conventional variable capacitor structure used in an LC tank VCO is shown in FIG. 1. As shown in this representation, four capacitors are individually controlled by each respective control bit b<3:0>. For example, each capacitor is individually set when control bits b<3:0> are set to either low capacitance (b<3:0>=1) or high capacitance (b<3:0>=0). The capacitors controlled by control bits b<3:0> are typically referred to as coarse tune capacitors and the capacitors controlled by the filter are called the fine tune capacitors. The voltages at nodes vfp and vfn, which are provided from a differential filter used in a pll loop, are used to control the fine tune capacitors. The coarse tune capacitors set the VCO frequency band and as the differential filter voltage changes the VCO frequency changes within the selected frequency band. With four coarse tune capacitors it can be seen that a total of 16 frequency bands are achieved.
FIG. 2 shows a typical VCO frequency variation with differential filter voltage for the design shown in FIG. 1. By way of explanation, the differential filter voltage is defined as: voltage at vfp−voltage at vfn. In the conventional design, the vfp voltage varies from 0.1V to 0.9V (for a supply voltage of 1V) and vfn voltage varies from 0.9V to 0.1V. The VCO gain, defined as the change in frequency for a given change in control voltage, changes with the frequency band as can be seen in FIG. 2.
The VCO gain changes more at the higher frequency bands, with frequency band 15 showing the most variation. This is because at the higher frequency band, the total capacitance is reduced (by controlling the coarse tune capacitors) so the fine tune capacitors have more effect. The gain is the slope of the curves shown in FIG. 2.
The gains are found to be:                at middle of frequency band:                    frequency band 0: gain˜1.5 Ghz/V            frequency band 15: gain˜3.0 Ghz/V                        at top of frequency band:                    frequency band 0: gain˜1.0 Ghz/V            frequency band 15: gain˜2.0 GHz/V.                        
These considerable gain changes make it more difficult to maintain the pll loop bandwidth. For example, if the VCO gain changes by a factor of 2, the loop bandwidth will change by the same factor. For applications such as Peripheral Component Interface Express (PCIE), where the pll loop bandwidth needs to be tightly controlled, the VCO gain variation poses a serious challenge and makes it more difficult to meet desired specifications.